1. Field of the Invention
The invention relates to a coherent optical communication receiver of an optical satellite communication system with two-laser optical transmitter. More particularly, this invention relates to a coherent optical communication receiver which can extract a microwave reference signal for tracking the laser""s frequencies coming from a two laser optical satellite transmitter.
2. Description of the Related Art
The data rate of the current microwave communication system is about several hundred Mbit/s. It cannot meet the demand of highly increased data amount in the future. In optical fiber communication system, the data rate has reached 40 Gbit/s. The transmission bandwidth is larger than that of the conventional microwave satellite communication system. In the future, to obtain a global high speed communication backbone network by linking the satellite communication and the optical fiber communication, the bandwidth of the current satellite communication system has to be extended. Thus, while adopting the optical communication technique in satellite communication, the problem of having insufficient bandwidth of the satellite microwave communication has to be resolved.
The optical frequency of a laser is easily affected by temperature, sometimes the frequency drifting of a laser occurs. When a light signal is emitted from the optical transmitter to the receiver, some impacts including the influence of weather, laser light beam and satellite vibration should be considered. Thus demodulating the light signal received by an optical receiver, for a conventional coherent optical communication system, the light carrier tracking ability is strongly affected by the light frequency drifting from the optical transmitter. In addition, the conventional satellite communication system, the satellite requires a power source (solar energy) to provide electric power to the microwave emitter and receiver. For the optical satellite communication system applying in two-laser light emitting, only a mirror is required. Of course, the signal can be covered by the receiver disclosed in this invention.
The invention provides a coherent optical communication receiver for a two laser emitting satellite coherent optical communication system. The coherent optical communication receiver can overcome the influence of weather, temperature, light beam spreading of the transmitter laser and satellite vibration with correctly tracking the signal.
The coherent optical communication receiver of the satellite communication system provided in the invention comprises a telescope, an automatic frequency control circuit, and a frequency shift keying demodulator. The telescope collect the light signal emitted from the satellite transmitter station. The automatic frequency control circuit is followed the telescope with an optical fiber to receive the optical signal transmitted from the telescope, and then the two optical light signal coming from the transmitter mixing with the light signal provided in the receiver end as output. The frequency shift keying demodulator is electrically connected to the automatic frequency control circuit to receive the mixed signal, for demodulating the data. The optical signal transmitted from the transmitter includes a modulated signal laser light with a local reference laser frequency signal for automatic frequency control. The photodetector in the automatic frequency control circuit can mix the modulated signal laser light frequency and the local oscillator light frequency coming from the transmitter and the local oscillator light frequency given by the receiver end to be a mixed microwave signal that comprises a microwave reference signal and a demodulated signal on demand. The automatic frequency control circuit uses microwave reference signal mixing in a microwave mixer for tracking the demodulated signal. With a frequency shift keying demodulator, the received demodulated signal is correctly demodulated to output a data.
In the automatic frequency control circuit, a local oscillating laser is used to enhance the system sensitivity. A light polarization controller is used to control the laser light polarization of the local oscillating laser to an optimal polarization status. A 1xc3x972 optical fiber coupler is used to connect the telescope and the local oscillating laser to the photodetector. The beat frequencies of the received light carriers locate at microwave band. A microwave broadband matching network circuit is electrically connected to the photo-detector for impedance matching of the microwave signals. A microwave mixer is electrically connected to the microwave matching network for mixing the the two microowave beat signals. The intermediate frequencies including the frequency shift keying (FSK) signals are fed to the frequency shift keying demodulator for receiving the baseband data. A frequency discriminator is electrically connected to the microwave mixer to discriminate the mixed intermediate frequency signal and output a discriminating signal. A low pass frequency filter is electrically connected to the frequency discriminator and the error signal feedback to the local oscillating laser for tracking the two laser"" frequencies.
In the invention, a frequency tracking signal generated from the distant dual frequency lasers is output from the optical transmitter, so that a heterodyne receiving can be performed between the very distant transmitter and receiver. Even with the consideration of the influence of satellite vibration and the attenuation through atmosphere, the quality of the satellite optical communication can be maintained. As the structure and fabrication of the receiver are simple, the fabrication cost can be reduced. This receiver is suitable for applying in a high speed satellite communication system. In addition, only put a mirror and a vibration control circuit on a satellite, no transponder is required on the satellite. Comparing with the microwave satellite system, the power consumption is thus reduced. The system is not affected by the sunspot and can provide multiple light beam superposition. Being reflected by a mirror, the multiple access can be achieved.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.